Imagine you and your friends are playing detective, and you have to figure out if someone did something wrong. You don't know for sure, but you want to gather clues and ask questions to see if they did it or not.
A grand-jury is a group of grown-up detectives, called jurors, who are asked to investigate a possible crime in a more serious way. They are usually made up of regular people from the community who are asked to serve for a certain amount of time.
The grand-jury men and women listen to evidence, just like you and your friends would look for clues, to decide if there is enough information to say someone did something wrong. They get to ask questions and hear from witnesses who might have seen something important.
If they think there is enough information to say someone did something wrong, they can decide to charge them with a crime. But if they don't have enough evidence or don't think the person did anything wrong, they can decide not to charge them with a crime.
So to put it simply, grand-jury men and women are like detectives who listen to evidence and decide if someone should be charged with a crime or not.